THE DEVELOPMENT OF MAMMALS 545 



resemble the embryos and not necessarily the adults of lower vertebrates. 

 Early vertebrates evolved a series of developmental stages that resulted 

 in their characteristic organs. Higher vertebrates have certain differen- 

 ces, but these develop by introducing changes in the later stages of 

 development rather than by altering the whole complex and intricately 

 interrelated development sequence. Development, therefore, tends to be 

 conservative, and the early embryos of different animals may bear 

 marked resemblances to each other. However, the early development of 

 an embryo may be altered and correlated with special conditions to 

 which the embryo has become adapted. The extraembryonic membranes 

 of mammals, for example, develop in advance of the main body of the 

 embryo, and the placenta is formed very early. This is an adaptation of 

 the embryo to intrauterine life. In reptiles the extraembryonic mem- 

 branes develop only after the body of the embryo is well established. 



A narrow band of mesoderm, known as the nephrogenic ridge, lies 

 between the somites and the lateral plate. This part of the mesoderm 

 differentiates into the kidney, as described in section 238, and helps 

 form the gonads. 



The entire circulatory system develops from the mesoderm, and its 

 development is rapid in all vertebrates. Transporting vessels are neces- 

 sary for the embryo to obtain nutrients from the placenta, or yolk, as 

 the case may be. The blood vessels differentiate by the hollowing out and 

 coalescence of cords and knots of mesodermal cells that appear first in 

 the mesodermal layer next to the yolk sac. A pair of vessels that are 

 destined to become the heart develop in the anterior part of the em- 

 bryonic disc before the neural tube is completely formed. Subsequent 

 foldings that give the embryo its shape carry these vessels beneath the 

 front of the embryo (Fig. 31.5). They fuse to form a single cardiac tube, 

 and the cardiac tube differentiates into the series of chambers found in 

 fish hearts (sinus venosus, atrium, ventricle, and conus arteriosus). Since 

 the cardiac tube grows in length faster than the part of the coelom (the 

 pericardial cavity) in which it lies, it folds and forms an S-shaped tube. 

 The atrium, which originally lay posterior to the ventricle, thus comes 

 to lie in front of the ventricle. Gradually the cardiac tube differentiates 

 into the adult heart. The atrium and ventricle become divided in mam- 

 mals, the sinus venosus is incorporated into the right atrium, and the 

 conus arteriosus forms part of the pulmonary artery and the arch of the 

 aorta. 



A series of paired aortic arches, which are similar in arrangement 

 to those of a fish, but are not interrupted by capillaries, carry blood 

 from the heart up through the pharyngeal region to the dorsal aorta. 

 Vitelline arteries extend from the aorta to the yolk, and umbilical 

 arteries follow the allantois to the chorion and developing placenta. 

 Veins develop in a similar manner, and return blood to the heart from 

 the yolk sac, chorionic villi and the embryo itself. In the early mam- 

 malian embryo, the pattern of the veins resembles the pattern seen in 

 fishes. Cardinal veins are present and the venae cavae do not develop 

 until later. The pattern of the circulation in a late fetus, and the changes 

 that occur at birth, were considered in section 236. 



